In a cellular radio telecommunications system, any number of multiple access strategies may be employed, such as, frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In a system that employs a FDMA strategy, the frequency spectrum is divided into a number of disjunctive frequency bands, wherein each frequency band serves as a separate radio channel. In a system that employs CDMA, different modulation codes, known as spreading codes, are used to distinguish the various radio channels. In a TDMA system, channel separation is accomplished by dividing the time domain into time frames and time slots as described in more detail below.
FIG. 1 illustrates a typical time division duplex (TDD), TDMA time frame 100. Generally, in a TDD-TDMA based system, the uplink channels and the downlink channels are separated in time over a common frequency. More specifically, FIG. 1 illustrates that the TDD-TDMA time frame 100 is divided into a number of fields including a downlink control channel (DL CC) field 105, a downlink traffic channel field 110, an uplink traffic channel field 115, and a random access channel (RACH) field 120. The downlink traffic channel field 110 and the uplink traffic channel field 115 are still further divided into a number of time slots (not shown). In the case of the downlink traffic channel field 110, a time slot is utilized for carrying information from the cell base station to an assigned mobile unit (e.g., cellular telephone) located in the cell. In the case of the uplink traffic channel field 115, a time slot is utilized for carrying information from an assigned mobile unit in the cell to the cell base station. Typically, the DL CC field 105 contains, among other things, an announcement list which identifies the mobile units that are to receive information from the base station in the current downlink period as well as the time slots during which the identified mobile units are to receive that information. The DL CC field 105 may also contain an assignment list which identifies those mobile units that have been allocated a time slot during the current uplink period. The RACH field 120 is typically utilized for transporting such information as scheduling information and control information, including retransmission requests.
FIG. 1 also illustrates that the TDMA time frame 100 includes an asymmetric uplink/downlink format. This means that the size (i.e., length) of the downlink traffic channel field 110 may be different than the size of the uplink traffic channel field 115. Moreover, this means that the relative size of the downlink traffic channel field 110 compared to the size of the uplink traffic channel field 115 may vary from cell to cell. Consequently, the time period associated with a downlink traffic channel field 110 in a first cell may, to some extent, overlap the time period associated with the uplink traffic channel field 115 in an adjacent or nearby cell, and vice versa.
When the downlink traffic channel field 110 in one cell overlaps the uplink traffic channel field 115 in an adjacent or nearby cell, the transmission of information in one cell is more likely to disturb the transmission of information in the other cell. That is, the overlap between the downlink traffic channel field 110 in one cell and the uplink traffic channel field 115 in the adjacent or nearby cell is more likely to result in cross-channel and/or co-channel interference. The situation is especially problematic wherein the two cells share (i.e., reuse) a portion of the frequency band, thereby increasing the likelihood of co-channel interference. As illustrated in FIG. 2, for example, if a base station 205 in a first cell is presently receiving uplink transmissions from mobile units located in the first cell, such as mobile unit 210, while a base station 215 in a second cell is presently transmitting downlink messages to various mobile units located in the second cell, such as mobile unit 220, the base station 205 in the first cell may inadvertently receive a transmission from the base station 215 in the second cell. In a second example, as illustrated in FIG. 3, the mobile unit 310 in the first cell may pick up transmissions from the mobile unit 320 located in the second cell.
The problem of intercell interference in a time division system is described in G. Povey et al., "TDD-CDMA Extension to FDD-CDMA Based Third Generation Cellular System", Proceedings of the ICUPC, IEEE (1997). Povey suggests synchronizing all of the cells in the network. In a large cellular network, however, this is an unrealistic and, most likely, a very complex proposal. Accordingly, there is a need to reduce and/or eliminate intercell interference due to asymmetric uplink/downlink traffic channel fields in a TDMA time frame.